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28 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Selecting Bridging, Switching, and
Routing Protocols


In this chapter we are going to look at bridging,
switching, and routing protocol attributes of:

Network Traffic characteristics

Bandwidth, memory, and CPU usage

The approximate number of peer routers or switches

The capability to quickly adapt to changes in an

The capability to authenticate route updates for security

Making Decisions as Part of the Top
Down Network Design Process

Factors involved in making sound

Goals must be established

Many options should be explored

The consequences of the decisions should be

Contingency plans should be made

Use a decision to match options with goals

Making Decisions as Part of the Top
Down Network Design Process (Cont’d)

Table 7
1 shows a decision table

Once decision is made look at it to determine:

What could go wrong

Hs it been tried before

How will customer react

Contingency plans if customer disapproves

Can use during both logical and physical design

Selecting Bridging and Switching

Decision making is simple because of few options

If includes Ethernet bridges and switches most likely use
transparent bridging with spanning
tree protocol

Might also need a protocol for connecting switches that
support virtual LANs

With Token Ring networks options include source
bridging (SRB), source
route transparent (SRT) bridging
and source
route switching (SRS)

Characterizing Bridging and
Switching Methods

Bridges operate at Layers 1 and 2 of OSI

Determine how to forward a frame based on
information in Layer 2 header

Bridge does not look at Layer 3 information

Bridge segments bandwidth domains so that devices
do not compete with each other for media access

Bridge does forward Ethernet collisions or MAC
frames in a Token Ring network

Characterizing Bridging and
Switching Methods (Cont’d)

Bridge does not segment broadcast domains. It sends
broadcast packets out all ports

Bridges normally connect like networks but can be a
translation or encapsulating bridge

A switch is like a bridge only faster

Switches take advantage of fast integrated circuits to
offer very low latency

Switches usually have a higher port density and a
lower cost per port

Characterizing Bridging and
Switching Methods (Cont’d)

Bridges do store and forward

Switches can be store and forward or cut

through is faster but more prone to
letting runts or error packets through

On a network that is prone to errors do not
use cut
through processing

Adaptive cut
through switching

Transparent Bridging

Most common Ethernet environments

A transparent bridge (switch) connects one
or more LAN segments so that end systems
on different segments can communicate
with each other transparently

Looks at the source address in each frame to
learn location of network devices

It develops a switching table (Table 7

Transparent Bridging (Cont’d)

Receives a packet look sup address in switch table

If no address it sends the frame out every port like
a broadcast frame

Send Bridge Protocol Data Unit (BPDU) frames to
each other to build and maintain the spanning tree

Sends BPDU to a multicast address every two

Route Bridging

Developed for Token Ring networks in the 80s by

Uses a source
transparent (SRT) standard

An SRT bridge can act like a transparent bridge or
a source
routing bridge depending on whether
routing information is included in a frame

Not transparent if pure SRB is used

Route Bridging (Cont’d)

Uses explorer frames

routes explorer

take all possible paths, take just
one route back

route explorer

takes just one path and
response take all paths or just one back

With single
route explorer frames the spanning
algorithm can be used to determine a single path

Scalability is impacted by amount of traffic when
routes explorer frames are used

Route Switching

SRS is based on SRT bridging

SRS forwards a frame that has no routing
information field

Learns the MAC addresses of devices on the ring

Also learns source
routing information for devices
on the other side of SRB bridges

Route Switching (Cont’d)


Rings can be segmented without adding new ring

can be incrementally upgraded to transparent bridging
with minimal disruption or reconfiguration

does not need to learn the MAC addresses of devices on
the other side of source
route bridges

can support parallel source routing paths

can support duplicate MAC addresses

Media Bridging

Mixture of Token Ring, FDDI and Ethernet bridging

Encapsulating bridging is simpler than translation
bridging but is only appropriate for some network

Encapsulating bridge encapsulates an Ethernet frame
inside an FDDI or Token ring frame for transversal
across a backbone network that has no end systems

Media Bridging (Cont’d)

Support for end systems on a backbone then
need to use translation bridging which translates
from one data
layer protocol to another


Incompatible bit ordering

Embedded MAC addresses

Incompatible maximum transfer unit (MTU) sizes

Handling of exclusive Token Ring and FDDI functions

No real standardization

Media Bridging (Cont’d)

While FDDI is a common choice for
backbone networks in campus network
designs to avoid translating Ethernet and
FDDI frames should use 100
Mbps Ethernet
or Gigabit Ethernet on backbone segments

Switching Protocols for
Transporting VLAN Information

When VLANs are implemented in a switched network
the switches need a method to make sure intra
traffic goes to the correct segments

Accomplished by tagging frames with VLAN

two tagging methods:

adaptation of the IEEE 802.10 security protocol

Switch Link (ISL) protocol

IEEE 802.10

A security specification used as a way of placing
VLAN identification (VLAN ID) in a frame

Inserted between the MAC and LLC headers of
the frame

The VLAN ID allows switches and routers to
selectively forward packets to ports with the same

VLAN ID removed from frame when forwarded
to destination segment

Switch Protocol

Another method for maintaining VLAN
information as traffic goes between switches

Developed to carry VLAN information on a 100
Mbps Ethernet switch
switch or switch
router link. Can carry multiple VLANs

ISL link is call a trunk. A trunk is a physical link
that carries the traffic of multiple VLANs between
two switches or between a switch and a router.
Allows VLANs to extend across switches

VLAN Trunk Protocol

Some networks have a combination of different
media types

VLAN trunk protocol (VTP) allows a VLAN to
span the different technologies by automatically
configuring a VLAN across a campus network
regardless of media type

VTP is a switch
switch and switch
VLAN management protocol that exchanges
VLAN configuration changes as they are made to
the network

Selecting Routing Protocols

A routing protocol lets a router dynamically
learn how to reach other networks and
exchange this information with other routers or

Selecting routing protocols is harder than
selecting bridging protocols because there are
so many

Made easier using a table such as 7
1 to pick
the best one

Characterizing Routing Protocols

General goal to share network reachability
information among routers

Some send complete other only an update

Differ in scalability and performance

Many are designed for small networks

Static environment

Some are meant for connecting interior campus

Vector Versus Link
State Routing Protocols

Two major classes: distance
vector and link

vector protocols

IP Routing Information Protocol (RIP) Version 1 and 2

IP Interior Gateway Routing Protocol (IGRP)

Novell NetWare Internetwork Packet Exchange Routing
Information Protocol (IPX RIP)

AppleTalk Routing Table Maintenance Protocol (RTMP)

AppleTalk Update
Based Routing Protocol (AURP)

IP Enhanced IGRP

IP Border Gateway Protocol (BGP) (path

Vector Versus Link
State Routing Protocols (Cont’d)

Vector means distance or course. A distance
vector includes information on the length of the
course. Many use hop count

A hop count specifies the number of routers that
must be traversed

Maintains a distance
vector routing table that
lists know networks and the distance to each.

Sends table to all neighbors, or an update after
first transmission

Vector (Cont’d)

Split Horizon, Hold
Down, and Poison

horizon technique

sends only routes that are
reachable via other ports

down timer

new information about a route to a
suspect network is not believed right away. A standard
way to avoid loops

reverse messages

way of speeding convergence
and avoiding loops. When a router notices a problem it
can immediately send a route update that specifies the
destination is no longer reachable

State Routing Protocols

Do not exchange routing tables

Exchange information about the status of their
directly connected links using periodic multicast

Each router builds its own routing table


IP Open Shortest Path First (OSFP)

IP Intermediate System
Intermediate System (IS

NetWare Link Services Protocol (NLSP)

State Routing Protocols

Converge more quickly

Less prone to routing loops

Require more CPU power and memory

More expensive to implement and support

Harder to troubleshoot

Routing Protocol Metrics

Used to determine which path is preferable
when more than one path is available

Vary on which metrics are supported

vector use hop count

Newer protocols take into account delay,
bandwidth, reliability and other factors

Metrics can effect scalability

Hierarchical Versus Non
Hierarchical Routing Protocols

Some routing protocols do not support hierarchy

Normally all routers perform same tasks

Hierarchical protocols assign different tasks to
different routers and group routers in areas

Some routers communicate with local routers in
the same area and other routers have the hob of
connecting areas, domains, or autonomous

Interior Versus Exterior Routing

Interior protocols, such as RIP, OSPF, and
IGRP are used by routers within the same
enterprise or autonomous

Exterior such as BGP perform routing
between multiple autonomous systems.

Classful Versus Classless
Routing Protocols

A classful routing protocol always considers
the IP network class

Address summarization is automatic by major
network number and discontiguous subnets are
not visible to each other

Classless protocols transmit prefix
length or
subnet mask information with IP network
addresses. The IP address can be mapped so
that discontinuous subnets and VLSM are

Dynamic Versus Static and
Default Routing

Static routes are often used to connect to a
stub network

A stub network is a part of an internetwork
that can only be reached by one path

Internal routers can simply be configured
with a default route that points to the ISP

Scalability Constraints for
Routing Protocols

Consider customer’s goals for scaling the
network to a larger size

There are a number of questions that relate
to scalability that should be answered

They can be answered by watching routing
protocol behavior with a protocol analyzer
and by studying the relevant specifications

Routing Protocols Convergence

Convergence is the time it takes for routers
to arrive at a consistent understanding of the
internetwork topology after a change takes

Understand the frequency of changes, links
that fail often, etc

Convergence time is a critical design

Routing Protocols Convergence

Convergence starts when a router notices a link
has failed

If a serial link fails it can start immediately. If
it uses keepalive frames it starts convergence
after it has been unable to send two or three
keepalive frames

If use hello packets and the hello timer is
shorter than the keep alive timer then routing
protocol it can start convergence sooner

IP Routing

Most common protocols are RIP, IGRP,
Enhanced IGRP, OSPF, and BGP

Routing Information Protocol

The first standard routing protocol developed for
TCP/IP environments

It is a distance
vector protocol that features
simplicity and ease

Uses a hop count to measure the distance to a
destination. Cannot be more than 15 hops

RIPv2 developed to address some of the
scalability and performance problems with
Version 1

Interior Gateway Routing

Meet needs of customers requiring a robust
and scalable interior routing protocol

Uses composite metric based on:
bandwidth, delay, reliability, and load

Load balances over equal
metric paths and
metric paths. (3 to 1)

Has a better algorithm for advertising and
selecting a default rout than RIP

Enhanced Interior Gateway
Routing Protocol

Meet the needs of enterprise customers with
large, complex, multiprotocol internetworks

Goal is to offer quick convergence on large
networks. Diffusing update algorithm (DUAL)
guarantees a loop
free topology

The router develops a topology table that
contains all destinations advertised by
neighboring routers. It can scale to thousands
of nodes

Open Shortest Path First

Open standard supported by many vendors

converges quickly

authenticates protocol exchanges to meet
security goals

supports discontiguous subnets and VLSM

sends multicast frames vice broadcast frames

does not use a log of bandwidth

can be designed in hierarchical areas

Open Shortest Path First (Cont’d)

Propagates only changes

accumulate link
state information to calculate
the shortest path to a destination

all routers run the same algorithm in parallel

Allows sets of networks to be grouped into

A contiguous backbone area, called Area ) is

Assign network numbers in blocks that can be

Border Gate Protocol

iBGP used at large companies to route
between domains

EBGP is often used to multihome an
enterprise’s connection to the Internet

Main goal is to allow routers to exchange
information on paths to destination

Apple Talk Routing

Three options:

Routing Table Maintenance Protocol (RTMP)

AppleTalk Update
Based Routing Protocol

Enhanced IGRP for AppleTalk

RTMP is most common because it is easiest to
configure and is supported by most vendors

Routing Table Maintenance

Routing table sent every 10 seconds using
split horizon

Works closely with Zone Information
Protocol (ZIP)

Checks routing table updates and sends ZIP

Using Multiple Routing and
Bridging Protocols

Important to realize you do not have to use
the same routing and bridging protocols
throughout the internetwork

To merge old networks with new networks
it is often necessary to run more than one
routing or bridging protocol

Solutions include source
route transparent
bridging, external routes in OSPF and RIP2

Redistribution between Routing

Redistribution allows a router to run more than
one routing protocol and share routes among
routing protocols

Network administrator must configure
redistribution by specifying which protocols
should insert routing information into other
protocol’s routing tables

A router can learn about a destination from
more than one protocol

Integrated Routing and Bridging

CISCO offers support for IRB which
connects VLANs and bridged networks to
routed networks within the same router

One advantage of IRD is that a bridged IP
subnet or VLAN can span a router


Deciding on the right bridging, switching,
and routing protocols for your customer will
help you select the best switch and router
products for the customer